1. Field of the Invention
This invention is in the field of electrochemical or galvanic batteries used to convert chemical energy into electrical energy and having means for providing cross-flow circulation of electrolyte in one direction through electrodes (U.S. Class 429/72, Int. Class H01M 2/36), to achieve accelerated chemical reaction rates that provide high electrical current and energy storage capacity.
2. Description of Related Art
Electrochemical cells for converting chemical energy into electricity (known as galvanic cells, which produce electrical energy) can be segregated into four classes based on their modes of storing and converting energy; namely:                Flow Cells (also known as Flow Batteries) that derive their energy from dissolution within the cells of externally stored binary electrolytes, such as a halide in a metal halide (e.g., chlorine in zinc chloride or bromine in zinc bromide). Flow cells may have one or two electrolytes. If either of the electrolytes passes through one electrode, it does not pass through the other. An example of a flow cell with one electrolyte is described in U.S. Patent Publication Nos. US 2010/0009243 and US 2010/0021805 A1 by Winter.        Fuel Cells that derive their reversible thermodynamic Gibbs free energy from external fuels, such as H2, and use electrolyte for mass transport of protons or anions (e.g., hydroxide ions) between electrodes to support three-phase electrode reactions (e.g., electrode catalyst, electrolyte, fuel). An example is described in Fuel Cells Comprising Laminar Flow Induced Dynamic Conducting Interfaces . . . , U.S. Pat. No. 7,252,898 issued on 7 Aug. 2007 to L. J. Markoski et al.        Pseudocapacitor cells (also known as supercapacitors, ultracapacitors and electrochemical double-layer capacitors) store redox reaction electropotential (voltage) electron or hole charges in a very thin electrolyte layer adjacent a solid phase faradaic material containing metal atoms that can change their atomic valance to acquire or free electrons or holes that move in the layer to form an electric double layer (EDL). The positive and negative charges travel very short distances only within their EDLs amongst shallow depths of solid phase atoms; but, not between electrodes. The first United States patent for a pseudocapacitor was No. 2,800,616 of 23 Jul. 1957 to H. I. Becker for a Low Voltage Electrolytic Capacitor. A more recent description is found in B. E. Conway, Electrochemical Capacitors—Scientific Fundamentals and Technological Applications, ©1999, Kluwer Academic/Plenum Publishers, New York, ISBN 0-306-45736-9.        Battery cells (also known as batteries, stacks or piles comprising one or more cells) store energy in their electrodes that is derived from chemical two-phase electrode redox reactions between faradaic materials (e.g., compounds or metallic states of iron, lead, lithium, nickel, silver, zinc, etc.) containing metal atoms that can change their atomic valance to acquire or free an electron or that can detach or acquire ions that are mobile in electrolytes. The ions must pass from within and between opposed polarity electrodes through the electrolyte while electrons from the redox reactions must simultaneously pass through an external electrical circuit. Electrolytes may be aqueous acid, alkaline or organic solvent or solute salts or metal cations that add or remove metal from electrodes. An example is described in Lithium-Ion Battery, U.S. Pat. No. 7,582,387 issued on 1 Sep. 2009 to W. G. Howard et al.Each of these four cells has attributes that make it better suited for a particular application than the other three cells.        
For example, flow cells excel at utility load averaging where excess utility energy can be used by balance-of-plant (BOP) to recharge electrolyte. Fuel cells excel at providing high energy output from external fuels that reside in compact containers needed for mobile applications. Pseudocapacitors are capable of emulating capacitors; but, can have two orders of magnitude higher capacity ratings in farads for meeting sudden load surges. Batteries do not charge or discharge as fast as pseudocapacitors; but, have vastly higher energy density ratings because their electrodes contain a much larger molar density of reaction-accessible faradaic material than those of pseudocapacitor EDLs and they can operate at higher voltages. Both attributes are a consequence of transferring ions between polar electrodes.
Energy density is expressed as watt-hours per cubic centimeter (wh/cm3) or Joules/cm3 or watt-hours per kilogram (wh/kg) or Joules/kg. Power density is measured in Watts/cm3 or Watts/kg. Charge and discharge rates are stated as Joules per gram-second (Joules/gm-second) or amperes per gram (Amps/gm). Charging rates are a function of the internal impedance of the battery and a maximum suitable voltage that does not cause electrode polarization or cell damage. Another common figure of merit is Amp-hr per kg or per liter, which relates more directly to battery capacity and is independent of voltage under load.
C is the total charge or coulomb storage capacity of a slowly-charged battery in Ampere-hours. C-rate is a common metric for grading how rapidly a battery can reach full rated charge or discharge capacity based on useful charge or discharge times; for example, in one-hour (1 C) at rated charge or discharge voltages. If a battery is charged or discharged at an accelerated or decelerated rate, then the rate is expressed as a multiple of kC, where k=(dis)charge rate/1 rate and C′/C is a capacity ratio that is dependent on battery type. C′/C as a function of k represents a battery performance metric and is a measure of a battery's ability to accept accelerated charge and discharge rates.
Galvanic batteries of this invention comprise porous electrodes infused with electrolyte. Electrochemists have long understood that exposure of electrolyte to highly porous electrodes will greatly improve overall efficiency of electrochemical systems. Newman et al, Electrochemical Systems, 3rd Ed., ©2004, Wiley Interscience, ISBN 0-471-47756-7, at Chapter 22—“Porous Electrodes” (pp. 517-565) presents analyses of porous electrodes suffused with electrolytes for two different types of electrochemical cells; namely, galvanic batteries (§22.4, pp. 535-551) and electrochemical reactors (§22.6, pp. 553-558), which require an input of energy to drive a reaction, used for recovery and removal of electropositive metals in solution. Only electrochemical reactors are described as comprising flow-through electrodes. There is no description of any galvanic battery or fuel cell employing a flow-through or a cross-flow electrolyte and porous electrode structure as described and claimed here.
A primary purpose in using such porous electrodes in any electrolytic cell or galvanic battery is to increase by several orders of magnitude the faradaic reaction surface exposed to electrolyte that fills its pores. A faradaic reaction occurs when an electric charge or mass participates in a charge-transfer reaction through an interface between dissimilar materials (e.g., an electrode-electrolyte interface). However, not all of the faradaic surface exposed to electrolyte may contribute to cell charge or discharge current.
Newman et al, at p. 538, FIG. 22.6, illustrates a pictorial or model of a prior art battery system containing a lithium foil anode, a cathode (positive electrode) and a separator interfaced between the two electrodes. The cathode is a substantially thick, porous volume containing a large faradaic material surface that is filled with electrolyte and has a current collector affixed to its end opposite of that nearest the separator. Thickness is measured in an orthogonal direction from the separator-cathode interface. The porous electrode is shown as divided into an unreacted zone adjacent the current collector and a reacted zone adjacent the separator.
The reacted and unreacted zones are divided from each other by a narrow reaction zone that is very much thinner than the thickness of the porous electrode. As the cell discharges, the narrow reaction zone moves toward the cathode's current collector so that the reacted zone becomes thicker and the unreacted zone becomes thinner. Movement of the narrow reaction zone increases the cell's internal resistance.
More important than internal resistance in limiting cell current is the width and position of the narrow reaction zone, which is very thin. The narrow reaction zone has a very small volume that reduces ionic mass transport rates and lowers electrolyte diffusion rates through the porous electrode. Because there are ionic species concentration gradients for anions (e.g., O2−, OH−) and cations (e.g., H+, Li+) in the porous electrodes, the position of the narrow reaction zone determines the number of ions available for completion of redox reactions. Both of these factors limit electric current in conventional cells and batteries.
One model of ionic species concentration as a function of position in a porous electrode is described in Newman & Tobias, Theoretical Analysis of Current Distribution in Porous Electrodes, J. Electrochemical Society, Vol. 109, No. 12 (December 1962), pp. 1183-1191. The authors caution that there is no consideration of capacitive effects of a double layer. This means that certain time-dependent processes, such as alternating current behavior and surge response, may not be accurately explained by the model. Nevertheless, the model highlights the spatial dependence of ion concentration and its effects on cell current. These limitations of ion mobility under diffusion, dispersion and migration gradients are common to all prior art galvanic cells with porous electrodes.
The total flux of a mobile ionic species in electrolyte is the vector sum of migration, dispersion, diffusion, and convection fluxes. Electrical current is proportional to the ionic species total flux per unit volume of electrode suffused with electrolyte rather than the reaction rate per unit of electrode cross-sectional area—also known as projected area. The flux value changes when species move, participate in an electrochemical reaction or change temperature. In conventional galvanic cells, diffusion, dispersion and migration fluxes have the greatest magnitudes, while the magnitude of convection flux ranges from very small to zero.